1. Field of the Invention
The present invention relates to gelled fracturing fluids of the type used in well bore operations and particularly to a method for producing a gradual reduction in the viscosity of a gelled fracturing fluid through the use of enzymes incorporated in the gelled fluid.
2. Description of the Prior Art
During hydraulic fracturing, a sand laden fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gellation of suitable polymers, such as a suitable polysaccharide. The gelled fluid can be accompanied by a propping agent which results in placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is referred to as "breaking" and can be accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
In addition to the importance of providing a breaking mechanism for the gelled fluid to facilitate recovery of the fluid, the timing of the break is also of great importance. Gels which break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature breaking can also result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture length in the fracture being created.
On the other hand, gelled fluids which break too slowly can cause slow recovery of the fracturing fluid from the produced fracture with attendant delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in at least partial closing and decreased efficiency of the fracturing operation.
For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured.
Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within a specific period of time after completion of the fracturing period. At higher temperatures, for example, about 24 hours is sufficient. A completely broken gel will be taken to mean one that can be flushed from the formation by the flowing formation fluids or that can be recovered by a swabbing operation. In the laboratory setting, a completely broken, non-crosslinked gel is one whose viscosity is either about 10 centipoises or less as measured on a Model 50 Fann viscometer Rl/Bl at 300 rpm or less than 100 centipoises by Brookfield viscometer spindle #1 at 0.3 rpm.
By way of comparison, certain gels, such as those based upon guar polymers, undergo a natural break without the intervention of chemical additives. The break time can be excessively long, however. Accordingly, to decrease the break time of gels used in fracturing, chemical agents are incorporated into the gel and become a part of the gel itself. Typically, these agents are either oxidants or enzymes which operate to degrade the polymeric gel structure.
However, obtaining controlled breaks using various chemical agents, such as oxidants or enzymes, has proved difficult. Common oxidants are ineffective at low temperature ranges from ambient temperature to 130.degree. F. The common oxidants require either higher temperatures to cause homolytic cleavage of the peroxide linkage or a coreactant to initiate cleavage. Common oxidants do not break the polysaccharide backbone into monosaccharide units. The breaks are nonspecific, creating a mixture of macromolecules. Further, common oxidants are difficult to control. They react with things other than the polymeric gel. Oxidants can react, for example, with the tubing and linings used in the oil industry as well as resins on resin coated proppants.
Using enzymes for controlled breaks circumvents the oxidant temperature problems. The enzymes are effective at the lower temperatures. The commonly known enzymes, however, are non-specific mixtures that are sensitive to higher pH which causes other problems. Enzymatic activity rapidly declines after exceeding pH 8.0 and denatures above pH 9.0. In the case of borate cross-linked guar gels, the gels are also pH dependant requiring pH in excess of 8.0 to initiate gellation. As the pH increases, the resulting gel becomes stronger, often requiring less borate crosslinker. Normally, when enzymes are used with borate crosslinked fluids these gels are buffered to maintain a pH range of 8.2 to 8.5 to ensure both gellation and enzymatic degradation. This technique requires high concentrations of both borate and enzyme. Unfortunately, while ensuring good breaks, the initial gel stability and proppant transport capability is weakened. The determination of the optimum enzyme concentration is a compromise between initial gel stability and an adequate break.
In the prior art systems, enzymatic degradation of the polymer improves at a lower pH range. By introducing another substance to the fracturing fluid, the pH can be raised for gellation, then lowered for enzymatic degradation. See, for instance, U.S. Pat. No. 5,067,566. This pH-regulating substance, for example, a low molecular weight ester, slowly hydrolyzes to produce a Bronsted acid, thereby dropping the pH of the fracturing fluid.
This technique, however, requires the introduction of another substance to the fracturing fluid. Adequate degradation depends on adequate hydrolysis of this pH-regulating substance. If distribution is uneven, the degradation may vary on local conditions within the gel.
Another problem with the lower pH ranges required for enzyme activation concerns proppant transport. Maximum proppant transport requires more alkaline pH levels. Higher pH levels increase the viscosity of the fluid during proppant transport than at a lower pH level. A higher viscosity keeps the cracks within the subterranean formation open better.
Conventional enzyme breaker systems generally degrade the gel polymers inadequately. These enzymes, for example, the cellulases, hemi-cellulases, amylases, pectinases, and their mixtures are familiar to those in the well service industry. These enzymes break the bonds that connect the monosaccharides into a polysaccharide backbone, for instance, the 1,4-.alpha.-D-galactosiduronic linkages in pectin. These conventional enzyme breaker systems are nonspecific and cause random breaks. As a result, these prior art enzyme systems only partially degrade the polysaccharide polymer. Instead of fragmenting almost completely into much smaller fragments such as monosaccharides, the enzymes break the polysaccharide gel into larger fragments consisting of a mixture of disaccharides, oligosaccharides and polysaccharides. These larger gel fragments have been shown to cause residue problems in the fractured formation once the fracturing operation is complete. Such residue decreases productivity by restricting the flow of fluid and plugging the formation.
The present invention has as its object to provide a break mechanism for a gelled fracturing fluid which yields high initial viscosity with little change during pumping but which produces a rapid break in the gel after pumping is completed to allow immediate recovery of the fluid from the formation.
Another object of the invention is to provide a gel system for a well fracturing operation which can break the gel polymers within a wide range of pH at low to moderate temperatures without interfering with the crosslinking chemistry.
Another object of the invention is to provide an enzyme breaker system which breaks the crosslinked polymer backbone into principally monosaccharide and disaccharide fragments.
Another object of the invention is to provide a gel breaker which does not require a mechanism to shift the pH for the desired enzyme breaker activity, allowing the pH to remain at higher levels for proppant transport.
Another object of the invention is to provide an enzyme breaker system for a gelled fracturing fluid which produces a controlled break over a wide pH range and at low temperatures and which decreases the amount and size of residue left in the formation after recovery of the fluid from the formation.